The present invention relates generally to the field of industrial combustion processes and in particular to a new and useful apparatus and method for removing contaminants from combustion gases prior to release into the atmosphere.
Fossil fuel combustion is used in industrial processes for many different purposes. Coal and natural gas are commonly burned to heat steam in electric power generation plants, for example. Unfortunately, fossil fuel combustion produces several contaminants which have been found to be detrimental to the environment. In particular, sulfur and nitrogen oxide compounds are major components of “acid rain”, which is harmful to plants.
In recognition of the harm caused by SOx and NOx compounds, different combustion gas cleaning systems have been developed to remove these components of combustion flue gases prior to release of the flue gases into the atmosphere.
Flue gas desulfurization systems are one such flue gas cleaning system. For a general description of the characteristics of flue gas desulfurization systems, the reader is referred to Chapter 35 of Steam/Its Generation and Use, 40th Edition, The Babcock & Wilcox Company, Barberton, Ohio, U.S.A., ©1992, the text of which is hereby incorporated by reference as though fully set forth herein.
Flue gas desulfurization systems and other liquid-gas contact processes have been designed and constructed using a perforated metal tray to produce and support a liquid-gas mixing or contact zone in which the vertically flowing gas passes up through the perforations as a liquid slurry or solution containing the reagent is falling down through the same perforations. An example of such a system is described in U.S. Pat. No. 4,263,021 for a “Gas-Liquid Contact System” assigned to the Babcock & Wilcox Company, which is hereby incorporated by reference as though fully set forth herein.
FIG. 1 herein illustrates the prior art flue gas cleaning system of U.S. Pat. No. 4,263,021. A gas, such a flue gas 40, is passed upwardly from inlet 55 at velocities of 5-20 feet per second through an upright tower 50 in counter-current contact with liquid, such as liquid slurry 65 which is introduced near the top through one or more spray headers 68 and discharged from the bottom of the tower. One or more horizontally disposed perforated plates, each forming a tray 60, is positioned intermediate the height of the tower 50. Each plate is provided with a plurality of upright partitions attached to the plate and arranged to subdivide the upper plate surface into a plurality of generally equal-area open-topped compartments.
With a proper coordination of liquid and gas flow rates, plate perforation arrangement and spacing dimensions, the gas and liquid will form gasified liquid masses in the compartments leading to stabilized liquid holdup encouraging both intimate contact and sufficient contact time for adequate chemical interchange between the media for absorption purposes. The cleaned gases 80 continue rising through tower 50 to mist eliminator 70 before exiting through outlet 75, while contaminants removed from the gases are disposed of with the discharged liquid.
A second known type of flue gas desulfurization system is illustrated by FIG. 2, in which horizontally flowing flue gas 40 is treated with slurry 65 introduced from headers 67 mounted in the top of the desulfurization chamber 51. The slurry 65 is essentially sprayed “cross-currently,” i.e. in cross flow, perpendicular to the flow of flue gas 40. The cleaned gas 80 leaves the chamber 51 after passing through mist eliminator 70 adjacent to outlet 75. Liquid slurry with contaminants is drained from the bottom or lower portion 53 of chamber 51 in any known manner.
These horizontal systems do not use a gas-liquid contact device such as the perforated tray as described above. Horizontal systems like that of FIG. 2 have been plagued with performance problems and limitations due to poor mixing of the gas and liquid. Stratification occurs where lighter flue gas seeks the top and heavier liquid reagent moves to the bottom of the reaction chamber without good mixing or sufficient contact time.
Horizontal flue gas desulfurization systems are sometimes required in retrofit applications due to space constraints. And, in new plants, a horizontal system is sometimes preferred for a variety of reasons including available space or height limitations.
Due to the harm caused by flue gas contaminants and the fact that a 100% efficient flue gas desulfurization system has not yet been created, there is always a need for improved cleaning systems which remove a greater fraction of contaminants from flue gases. Further, systems which are more cost efficient to manufacture and more easily retrofit into existing fossil fuel combustion plants are highly desirable. A more effective horizontal flue gas desulfurization system is very desirable due to the lag in effectiveness between horizontal and vertical systems.